Teach DSM registry to ERROR if attaching to an uninitialized entry.

Teach DSM registry to ERROR if attaching to an uninitialized entry.

If DSM entry initialization fails, backends could try to use an
uninitialized DSM segment, DSA, or dshash table (since the entry is
still added to the registry).  To fix, keep track of whether
initialization completed, and ERROR if a backend tries to attach to
an uninitialized entry.  We could instead retry initialization as
needed, but that seemed complicated, error prone, and unlikely to
help most cases.  Furthermore, such problems probably indicate a
coding error.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/dd36d384-55df-4fc2-825c-5bc56c950fa9%40gmail.com
Backpatch-through: 17